An ethylene/tetrafluoroethylene copolymer (hereinafter sometimes referred to as “ETFE”) is excellent in heat resistance, weather resistance, electrical insulation properties, non-tackiness, water/oil repellency, and so on, and in particular, it is characterized in that it has high moldability and mechanical strength among fluororesins. Thus, a broad range of molding processed products including covered electric wires, tubes, sheets, films, filaments, pump casing, joints, packing, lining and coating have been manufactured by molding ETFE by a melt molding method such as extrusion, blow molding, injection molding or rotational molding (for example, Non-Patent Document 1).
In recent years, ETFE molded products become small in size, complicated and thin, and demands for cost reduction by improvement in productivity by high speed molding are increasing. To meet such requirements, ETFE is required to have a higher melt flowability.
To increase the melt flowability of ETFE, it is effective to decrease the molecular weight of ETFE. However, if the molecular weight of ETFE is decreased, the heat resistance, the stretch crack resistance, etc., characteristic to ETFE, tend to be low.
Patent Document 1 discloses that a fluorinated copolymer composition having excellent moldability can be obtained without decreasing mechanical strength of a molded product, by blending an ETFE having a low melt viscosity and an ETFE having a high melt viscosity.
Patent Document 2 discloses that ETFE having excellent stress crack resistance can be obtained by controlling the polymerization rate of ethylene and tetrafluoroethylene.
However, both of the high speed moldability of a fluorinated copolymer composition, and the heat resistance and the stress crack resistance of a molded product, cannot simultaneously be satisfied by the methods disclosed in Patent Documents 1 and 2.
Further, Patent Document 3 discloses that by adding an effective amount within a range of at most 10 parts by weight of copper oxide per 100 parts by weight of ETFE, a fluorinated copolymer composition excellent in thermal stability can be obtained without coloring or foaming even exposed to the air atmosphere at a temperature of at least 340° C. for a long period of time.
However, the thermal stability in Patent Document 3 means thermal stability against the heat history at the time of molding, for the purpose of preventing decomposition, foaming, coloring and the like of ETFE at the time of molding, and improvement of the heat resistance and the stress crack resistance of a molded product are not studied at all.